The present invention relates generally to a system for analyzing the operational performance of a ship. Accurate knowledge of three measured quantities is necessary to analyze the performance of a ship at any point in its lifetime. These quantities are ship speed, propulsion shaft torque and the revolving speed of the shaft. The product of torque and revolving speed is known as the shaft horsepower. Typically, the ship is designed to achieve a certain speed through the water at a certain shaft torque and speed in revolutions per minute (RPM). These quantities are measured during a trial trip of a newly built vessel to demonstrate that performance objectives have been met before the vessel is delivered to its owner. In practice, the results are usually expressed as speed in knots, shaft horsepower, and RPM.
During the life of the ship, changes may occur which affect the performance as measured by these three quantities. The three quantities are, in general, functions of each other and a change in one will result in a change in at least one of the others. The following are three possible changes in performance:
A. Ship speed and RPM both decrease at the same horsepower. This is caused by an increase in ship resistance.
B. Ship speed decreases and RPM increases at the same horsepower. This is caused by damage to the propeller.
C. Ship speed decreases, RPM decreases, and horsepower also decreases. This is caused by a malfunction of the power plant.
Measurement of the shaft speed in revolutions per minute by means of a shaft revolution counter is commonplace and available on virtually all vessels.
Measurement of shaft horsepower accurately is seldom found on vessels in normal commercial service except during the trial trip at which time the builder installs a special testing instrument for this purpose, that instrument being removed after the trial trip. In recent years advances have been made in propulsion shaft torsionmeters, and also it is not now uncommon for such devices to remain onboard after the trial trip.
The most difficult of the three parameters to measure accurately is the ship speed, primarily because deep water is necessary to avoid shallow water effects upon speed and there is typically a lack of a good physical sighting reference in deep water. Several years ago a method of speed measurement was developed using the Loran system, a radio aid to navigation. This method of speed measurement utilizes successive position fixes, and has been successfully used for trial speed measurement on several occasions. The heart of the method depends upon the errors in Loran being of a random nature, and hence their effects subject to treatment by the use of statistical mathematics.
Loran is a hyperbolic navigation system which is based on the fact that the transmission time of a radio pulse traveling a certain distance is a measure of that distance. A pulse travels 983.24 feet or 0.16171 nautical miles in one microsecond (millionth of a second), and therefore measurements of time of receipt of signals can readily be converted into distances from transmitting stations.
A pair of Loran transmitting stations located several hundred miles apart emit pulses at certain intervals, and a navigator by means of a Loran receiving unit can measure the time difference in receipt of signals from the two stations. The Loran receiving unit is essentially an electronic stop watch which measures accurately in microseconds this time difference. In practice a "Master" transmitting station emits a signal which is received a fixed interval of time later at a "slave" station and which triggers, after a fixed time delay, a pulse from the slave station. The navigator aboard a ship or airplane in the area measures the time difference between his receipt of the "Master" signal and the "slave" signal. Since the time difference is a measure of distance, it follows that the vessel or airplane lies on a line of constant difference in distance from the two stations. These lines of constant time or distance difference are a family of hyperbolae with the stations as foci. A line of position may therefore be determined by a single reading utilizing one station pair, and a "fix" may be obtained by measuring the time difference in receipt of signals from a second pair of stations.
Loran lines are closest together on a base line between stations, and therefore most accurate measurements of speed or distance can be made by steaming on or close to a Loran base line and on a course such that the Loran hyperbolae are crossed at right angles.
In order to measure speed by the use of Loran, there are three factors which must be determined. The vessel's speed during a run must be obtained in microseconds per unit of time, the vessel's course over the ground must be determined, the conversion factor of microseconds to nautical miles in the area in which the run is made must be known. Each of these factors must be determined to an accuracy such that the product of the three will be of at least the required accuracy.
In order to limit the length of speed runs to a reasonable distance, it was recognized that the methods of statistical mathematics must be applied to a rather large number of Loran readings if the desired accuracy were to be attained. It has been demonstrated that the errors in Loran readings are random errors so that statistical techniques could be applied to attain the required accuracy. In particular, the Method of Least Squares has been used to eliminate the effects of the random errors in Loran readings.
One refinement which is not necessary to the method but which has greatly contributed to ease of application and rapidity of answer is the use of a computer ashore. After each run, the computer facility was contacted by radio phone and given the observed Loran data. Upon completion of the next run the answers for the first run complete with a statistical analysis and a check for random error validity were available on the phone.
One basic limitation to the use of Loran in the past has been the requirement that the ship run as perpendicular as possible to one set of Loran lines. This limitation was required to enable a reasonably accurate determination of the scale factor for converting Loran microsecond readings to nautical miles. The present invention relates to an improvement wherein the ship's speed may be calculated while the ship is running in any direction, thereby eliminating the prior art constraint that the ship run as perpendicular as possible to one set of Loran lines.
A complete test on a ship requires correction for the effects of currents and winds on the vessel. This has been accomplished by averaging the data for runs at substantially the same speed over the same course but in opposite directions. The ship is run first in one direction at a heading substantially perpendicular to a set of Loran lines, and set of readings are taken along that run. The ship then turns and proceeds back in the opposite direction over the same course during which time a second set of readings are taken. The ship then reverses course and proceeds over the same track (and in the said direction as the first run) during which time a third set of readings are taken. This completes the test. To average out the effects of winds and currents, the data from the second run is utilized twice, and averaged against the data for the first and third runs.
With this in mind, it may be seen that data may be taken while the ship is proceeding in any direction rather than compelling it to run perpendicular to Loran lines. The data for the first run may now be taken while the ship is on its normal commercial course. The ship then need only turn 180.degree. for the second run, and once again to its original course during which the third set of readings is taken. A complete trial taken in this "heading independent" fashion, will result in a savings of up to several hours over the former method. When the hourly cost of running a ship is considered, it may be seen that a difference in several hours will result in substantial financial savings.
Further, an onboard permanent Ship Performance Analyzer enables the performance of the ship to be checked on a regular basis such that if there is a deterioration in performance, it may be readily detected and promptly corrected. Such an installation has the ability of performing a trial trip standardization at any time in the vessel's life without special manning or significant diversion from the vessel's commercial course. This should enable the ship during its life time to deliver the performance of which it is capable, and should result in the savings of substantial amounts of money, both in terms of a decrease in fuel used and an increase in cargo miles.